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Talk about the difference between TOD, BOD and COD

by:BOQU     2023-03-22
There are two types of comprehensive indicators indicating the content of organic matter in water. One is the index expressed by the oxygen demand (O2) equivalent to the amount of organic matter in water, such as biochemical oxygen demand (BOD), chemical oxygen demand (COD), and total oxygen demand (TOD). ; The other is an indicator expressed by carbon (c), such as total organic carbon TOC. For the same kind of sewage, the values ​​of these indicators are generally different, and the order of arrangement according to the value is TOD>COD>BOD5>TOC. 1. Total Oxygen Demand TOD Total Oxygen Demand TOD refers to the amount of oxygen required when reducing substances in water burn at high temperature and become stable oxides, and the result is expressed in mg/L. The TOD value can reflect the oxygen consumed when almost all organic substances in water (including carbon C, hydrogen H, oxygen O, nitrogen N, phosphorus P, sulfur S, etc.) become CO2, H2O, NOx, SO2, etc. after combustion quantity. 2. Total organic carbon TOC TOC is a comprehensive indicator that indirectly indicates the content of organic matter in water. The data displayed is the total carbon content of organic matter in sewage, and the unit is expressed in mg/L of carbon (c). The TOC of general urban sewage can reach 200mg/L, the TOC of industrial sewage has a wide range, and the highest can reach tens of thousands of mg/L. The TOC of sewage after secondary biological treatment is generally<50mg/L. 3. Biochemical Oxygen Demand (BOD) Biochemical Oxygen Demand is called Biochemical Oxygen Demand, abbreviated as BOD, which means that at a temperature of 20°C and aerobic conditions, aerobic microorganisms decompose the dissolved organic matter consumed in the process of biochemical oxidation in water The amount of oxygen, that is, the amount of oxygen required for the stabilization of biodegradable organic matter in water, is expressed in mg/L. BOD includes not only the oxygen consumed by the growth and reproduction of aerobic microorganisms or respiration in water, but also the oxygen consumed by reducing inorganic substances such as sulfide and ferrous, but the proportion of this part is usually very small. Under natural conditions at 20°C, the time required for organic matter to oxidize to the nitrification stage, that is, to achieve complete decomposition and stability, is more than 100 days. In production and application, 20 days is still considered too long, and the biochemical oxygen demand BOD5 of 5 days at 20°C is generally used as an index to measure the organic matter content in sewage. 4. Chemical Oxygen Demand COD Chemical oxygen demand COD refers to the amount of oxidant converted into oxygen consumed by the action of organic matter in water and strong oxidant under certain conditions, expressed in mg/L of oxygen. When potassium dichromate is used as the oxidant, almost all (90% to 95%) of the organic matter in the water can be oxidized. At this time, the amount of the consumed oxidant converted into oxygen is commonly referred to as the chemical oxygen demand, often abbreviated as CODcr. The CODcr value of sewage not only includes the oxygen consumption for the oxidation of almost all organic substances in the water, but also includes the oxygen consumption for the oxidation of reducing inorganic substances such as nitrite, ferrous salt, and sulfide in the water. 5. The relationship between BOD5 and COD BOD5 is not only an important water quality index, but also an important control parameter in the process of sewage biological treatment. However, due to the long measurement time (5d), it cannot reflect and guide the operation of sewage treatment equipment in time, so it can only be used for process effect evaluation and long-term Wangyi regulation. For a specific sewage treatment plant, the correlation between BOD5 and COD can be established, and COD can be used to roughly estimate the BOD5 value to guide the adjustment of the treatment process. Sometimes because some production wastewater does not have the conditions for microbial growth and reproduction (such as the presence of toxic organic substances), its BOD5 value cannot be accurately determined. The chemical oxygen demand (COD) value of laboratory sewage can determine the content of organic matter in water more accurately, but the chemical oxygen demand (COD) cannot distinguish biodegradable organic matter from non-biodegradable organic matter. People are accustomed to using the BOD5/COD of sewage to judge its biodegradability. It is generally believed that if the BOD5/COD of sewage is greater than 0.3, it can be treated by biodegradation. If the BOD5/COD of sewage is lower than 0.2. Only other methods can be considered. The COD value of chemical oxygen demand is generally higher than the BOD5 value of biochemical oxygen demand, and the difference between them can roughly reflect the content of organic matter in sewage that cannot be degraded by microorganisms. For sewage with relatively fixed pollutant components, there is generally a certain proportional relationship between COD and BOD5, which can be calculated from each other. In addition, the measurement of COD takes less time. According to the national standard method of reflux for 2 hours, it only takes 3~4 hours from sampling to the result, but it takes 5 days to measure the BOD5 value. Therefore, in the actual operation and management of sewage treatment, COD is often used as a control indicator. In order to guide production and operation as soon as possible, some sewage treatment plants have also formulated enterprise standards for measuring COD by reflux for 5 minutes. Although the measured results have certain errors with the national standard method, the errors are systematic errors and the results of continuous monitoring can accurately reflect water quality. The actual change trend, but the measurement time can be reduced to less than 1h, which provides a time guarantee for timely adjustment of sewage treatment operating parameters and preventing water quality mutations from impacting the sewage treatment system.
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